The sea water is salty as the result of many natural influences on it over the years.
Roughly about 3.8 million years ago, there were massive volcanic eruptions everywhere on earth. Consequently, large ash clouds began to form up in the sky. These clouds along with water vapor also contained an essence of the volcanic eruptions of the region that they were in. The rainwater coming from these clouds was acidic in nature and readily reacted with the rocks as it fell onto the ground.
The acids present in rainwater broke down heavy rocks, as a result of which, several salty ions and electrically charged particles were created. These ions and particles were collected in the sea water by a steady stream of rivers or lakes gushing into it, which made it salty. However, this is just one part of the answer.
Sea water is salty because of some other factors as well, such as the solid and gaseous materials that trickled down the earth’s crust into the seas. Another factor is the creation of volcanic vents which transported the lava and other volcanic materials into the seas. Decaying of biological matter in the sea also releases certain salts and salt compounds. Besides these, other natural processes such as soil erosion, weathering down of mountains, and mineral washings being flown into the seas have also caused sea water to be salty.
Presently, all the rivers and lakes of the world carry approximately 4 billion tones of salt into seas and oceans each year. Taking this count into consideration, the estimated salinity of seas and oceans comes to be about 3.5 percent. In other words, for every thousand parts of sea water, there is at least 35 parts of salt present in it.